Hydrogen fluoride is industrially produced by reacting fluorite with sulfuric acid in such a manner as shown in the following reaction formula. In this process, the hydrogen fluoride is recovered as a gas. EQU CaF.sub.2 +H.sub.2 SO.sub.4 .fwdarw.2HF+CaSO.sub.4
In this step, a part of SiO.sub.2 component contained as an impurity in the starting fluorite is reacted with the produced hydrogen fluoride to produce a silicon tetrafluoride gas in accordance with the following reaction formula. EQU SiO.sub.2 +4HF.fwdarw.SiF.sub.4 +2H.sub.2 O
The hydrogen fluoride gas thus produced contains not only silicon tetrafluoride but also other impurities such as sulfur dioxide, sulfuric acid and dusts, and is therefore purified by washing with sulfuric acid and then subjecting to condensing, rectifying and stripping. The silicon tetrafluoride gas is separated as a low boiling material from hydrogen fluoride in the rectifying step, and is withdrawn from the top of a rectifying column. The silicon tetrafluoride thus withdrawn and an accompanying hydrogen fluoride are absorbed with water, and is reacted to produce a silicofluoric acid solution as shown in the following reaction formula. EQU SiF.sub.4 +2HF.fwdarw.H.sub.2 SiF.sub.6
The gas discharged from the top of the rectifying column is adjusted to contain hydrogen fluoride in an amount larger than the equivalent amount of the above reaction in order to prevent blocking by SiO.sub.2 scale formed in a gas absorbing column. Thus, the composition of the aqueous solution having silicon tetrafluoride absorbed generally comprises from 30 to 50% by weight of H.sub.2 SiF.sub.6 and from 5 to 20% by weight of HF.
This solution is neutralized with Ca(OH).sub.2, and is then filtered to obtain a waste containing a mixture of CaF.sub.2 and SiO.sub.2 or to be used as a starting material for producing Na.sub.2 SiF.sub.6 by reacting with NaOH.
However, when the above-mentioned solution is neutralized with Ca(OH).sub.2, there is a problem that fluorine in this solution is disposed without being effectively used. Also, when the above-mentioned solution is reacted with NaOH to produce Na.sub.2 SiF.sub.6, it is not preferable to convert the total amount of this material into Na.sub.2 SiF.sub.6 since the demand for Na.sub.2 SiF.sub.6 is limited.